Previously, JS_ASYNC_GENERATOR_OBJECT_TYPE maps led to an UNREACHABLE
macro, but are now restored like ordinary JSGeneratorObjects.

BUG=chromium:772649, v8:5855
R=adamk@chromium.org, yangguo@chromium.org, verwaest@chromium.org

Change-Id: I02e101565625f8a057d0e5b242a5fe0df263df89
Reviewed-on: https://chromium-review.googlesource.com/706780
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48423}